Boost is the inches MP above or below ambient in the induction manifold. The more familiar MP gauge measures actual pressure in the manifold. They're both measuring the same thing - it's just the reference point that changes.
With the engine not running the boost would be zero. In a non-turbo engine it would always be at least slightly negative (think about a 'normal' MAP gauge. When does it ever go above ambient?). In a forced induction engine the boost would go above zero when the turbo- or super-charger is compressing the induction charge.